Nuclear technology was once the sole province of major powers. But nuclear know-how has now become more widely available, sparking concerns about the spread of nuclear weapons. Iran and North Korea each present different challenges in the fight against nuclear proliferation.

Persuading a nation not to embark down the nuclear path - or, if it already has, to retreat from it - is, say analysts, incredibly difficult, especially if the governments involved are internationally isolated and suspicious, like North Korea and Iran.

Kenneth Katzman, senior analyst on Middle East affairs at the nonpartisan Congressional Research Service, says each country requires a different approach to non-proliferation.

"The goal is the same in both," said Kenneth Katzman. "But the goal in North Korea's case is to roll back what everybody already agrees is an acquired nuclear weapon. That's a very hard task. To get a country that everybody believes has a nuclear weapon to give it up is extremely difficult. With Iran, it seems to me that there are more ways this could play out because Iran does not, everybody agrees, Iran does not have that device yet."

After more than two years of talks between North Korea and five other nations, North Korea agreed to give up its nuclear programs and allow United Nations inspectors in. In return, the five nations - which include the United States, China, Japan, Russia, and South Korea - promised to provide a civilian light water reactor and a pledge that they will not attack North Korea. But many contentious issues were left unresolved in the agreement in principle and must still be worked out.

But another nuclear agreement reached with North Korea in 1994 collapsed in 2002 after the United States accused Pyongyang of a secret program to enrich uranium, a step toward building a bomb. North Korea restarted two dormant reactors, booted out U.N. monitors, and pulled out of the nuclear Non-Proliferation Treaty.

Daniel Poneman, former senior director for non-proliferation at the U.S. National Security Council, says the collapse of the 1994 pact should not necessarily doom this new agreement.

"The October '94 agreed framework I don't think was predestined to fail," said Daniel Poneman. "And I think it bought us about eight years of zero plutonium out of a plutonium-able country, which I think is a big deal and a useful thing. So I would say if we could get another agreement now that would buy us another seven or eight years, this time perhaps of no plutonium and no highly enriched uranium, that would be a good thing."

Until recently the United States and Europe had had different approaches to Iran. The European Union was holding three-party talks with Iran, offering economic and political concessions if Iran would drop any nuclear ambitions. Iran has insisted it only wants nuclear power for peaceful energy purposes. The Bush Administration has insisted that is only a cover to pursue nuclear weapons and has been seeking to haul Iran before the United Nations Security Council for possible sanction.

The advent of a new, hardline leadership in Tehran in June under President Mahmood Admadinejad persuaded the Europeans to back the U.S. approach at a recent meeting of the International Atomic Energy Agency in Vienna. However, other countries, such as Russia and China, have balked at taking a tough stand with Iran.

Mr. Katzman says the reluctance of some nations to back a tough line with Iran is because Iran has a significant card to play that North Korea does not.

"The problem is, Iran has oil, and North Korea doesn't," he said. "And oil is again knocking at the gates of $70 a barrel. So that does give Iran some leverage that North Korea perhaps does not have."

Mr. Poneman believes it is better to work out nonproliferation agreements with North Korea or Iran, rather than what he says is a risky strategy of taking the offending nation to the United Nations.

"That's in my view always going to be preferable than going down a very uncertain route in the U.N. Security Council in which you don't know what kind of resolution you're going to get, you don't know how effective the implementation will be, and you don't know how efficacious it will be in driving the target country to come back into compliance with your preferences," he said.

Iran is believed to be still five to 10 years away from having any working nuclear weapon. Mr. Poneman says it is always easier to try to convince a government that does not have nuclear weapons to give up trying to get them than it is to persuade one that has atomic bombs to surrender them.